Yes, this is for real. It’s a sign posted on the fence at the Bison Paddock in Golden Gate Park (San Francisco CA USA). At least it was there last year.
No, I have no idea why the sign is there, although I suppose it makes sense to avoid giving bison psychoactive substances. Except maybe when prescribed by their vet?
The bison have been there since 1930, before San Francisco had a zoo. This small herd has remained in the park rather than relocating to the zoo at the south end of The Great Highway.